THE STEM 



185 



compound and primitive rays. In shrubby representatives of 

 the genus Vitis occurring in the southwest region of the United 

 States there is sometimes present the same mode of organization 

 of the rays as characterizes the stem of Leea figured above. In 

 more northern species of Vitis, however, a very different situa- 

 tion presents itself namely, that found in Fig. 138. Here the 

 only rays present are of the compound type. An interesting 



FIG. 138. Diagram of the organization of the stem in a northern species of Vitis 



light is thrown on the situation by the consideration of the seed- 

 ling in the genus Vitis. Fig. 139 presents the stem of the young 

 individual. There are seen masses of wood representing leaf 

 traces still in position in the woody cylinder of the stem. The 

 segments under consideration are clearly bounded on either side 

 by compound rays, while in the wood are present distinct vestiges 

 of primitive rays. It is evident that, if the anatomy of the seedling 

 has any clear bearing on the problem of the origin of the type of 

 stem presented in our northern species of grapevine, primitive rays 

 were once present in the intervals of wood bounded laterally by 

 the large or compound rays, and that these have subsequently been 



